Reading about writer's cap (5)
I believe the answer is:
limit
'cap' is the definition.
(I've seen this before)
'reading about writer's' is the wordplay.
'reading' becomes 'lit' (abbreviation for literature).
'about' is an insertion indicator.
'writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'lit' going around 'im' is 'LIMIT'.
(Other definitions for limit that I've seen before include "Utmost" , "Cap" , "Constrain" , "verge" , "Point beyond which one should not go" .)